For internal combustion engines, such as diesel engines, nitrogen oxide (NOx) compounds may be emitted in the exhaust. To reduce NOx emissions, a selective catalytic reduction (SCR) process may be implemented to convert the NOx compounds into more neutral compounds, such as diatomic nitrogen and water, with the aid of a catalyst and a reductant. The catalyst may be included in a catalyst chamber of an exhaust system, such as that of a vehicle or power generation unit. A reductant may be typically introduced into the exhaust gas flow prior to the catalyst chamber. To introduce the reductant into the exhaust gas flow for the SCR process, the reductant is introduced through a dosing module (doser), which inserts (e.g., injects) the reductant into an exhaust pipe of the exhaust system upstream of the catalyst chamber. The injector rest period and the injection event together comprise an injection cycle. The SCR system may include one or more sensors to monitor conditions within the exhaust system.
Reductants and/or reductant precursor formulations include solids, gases, and liquids. Examples of solid reductant precursor formulations and/or carriers include ammonium salts and metal ammines. In solid-state reductant delivery systems, a cartridge or canister with a solid material, such as an ammonia precursor or a substance with absorbed ammonia, is carried onboard. During engine operation, the carrier material is heated to release ammonia gas, which is metered into the exhaust gas. Examples of gaseous reductant formulations include gaseous ammonia, ammonia-air mixtures, and ammonia-nitrogen mixtures. Examples of liquid reductant formulations include aqueous ammonia and automotive-grade urea such as AdBlue®, BlueTEC™, and other Diesel Exhaust Fluid (DEF) products. An example ISO 22241-1:2006 DEF is an aqueous urea solution including 32.5% urea and 67.5% deionized water. The urea in the DEF turns to ammonia when heated. The ammonia reacts with the NOx in the exhaust stream to chemically reduce the NOx to water (H2O) and nitrogen (N2).
A pump may be used to pressurize the reductant for delivery from the reductant source to the dosing module and/or an SCR catalyst. Problematically, actual reductant delivery and dosing rates may vary from the expected reductant delivery and dosing rates in reductant delivery systems, which affects the accuracy and precision of the reductant flow and results in emission spikes due to fluctuations of the reductant dosed into the system. First, errors in flow rate may occur due to deviation of pressure from target during the pressure recovery period. Limitations to shortening the recovery period include mechanical lag and lack of accumulation at the point of injection. Second, even where it is possible to determine the error in flow rate, the injection cycle of the dosing unit may be too short to apply any real-time correction. These factors can result in emission spikes over time due to difference between expected and actual dosing quantity.
According to one conventional approach to error correction in reductant dosing using liquid (DEF) injection, pressure at the start of a reductant injection period can be recorded and used for correction to target pressure. However, a drawback associated with this approach is that the recorded pressure at the start may not be representative of the average pressure throughout the dosed period. Even if the pressure does recover to target, it may be impossible to determine if the pressure at the start of injection is the same as the target pressure. Additionally, the error during recovery is not accounted for. According to another conventional approach to error correction focused on liquid DEF injection, the delivered amount of DEF is allowed to accumulate over a prescribed sampling interval until the requested reductant quantity is reached within the specified injection period. However, a drawback associated with this approach is its underlying assumption that it is possible to dynamically make this determination on time within the current injection cycle.